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APPENDIX B 
WAFER CLEANING

Two methods are used for cleaning wafers in the ECE 344 lab. The first presented here is ultrasonic vapor degreasing. The other is the industry standard RCA clean. Both procedures are posted in the wet lab area so you do not need to take your copy of this into the area. Not only is space limited, but paper is very dirty and dusty by semiconductor industry standards. There is no point in carefully cleaning a wafer if it is not kept in a clean environment. Use your individual wafer carriers to keep your wafer clean and safe from accidents after either of these cleaning procedures.

I. B.1 DEGREASING PROCEDURE*

The term degrease refers to the removal of the grime that often coats surfaces exposed to the atmosphere. The thin film is mostly organic in nature and is probably due to the presence of humans. 1,1,1 Trichloroethane (TCA) is particularly effective in dissolving this "grease" which is why it is commonly used in industry. Substitutes must be found, however, since compounds containing halogens (chlorine, fluorine, and bromine) are destroying our ozone.

TCA is boiled in the left sump of the degreaser. Clean TCA boils at 162 F. Contaminants will raise this temperature. When cool wafers are placed on the screen above the boiling TCA, distilled quality TCA condenses on the wafers. The fumes are contained within the degreaser by cooling coils which you'll see dripping TCA into a trough. The trough drains into the rightmost sump which is for water separation. TCA must go under a baffle and past an additional cooling coil. Since water and most other likely contaminants are lighter than TCA, they can't get past the baffle. A TA will periodically drain off the top layer which may contain water. The water separation sump overflows into the middle sump which has ultrasonic transducers attached to it. Ultrasonic energy can mechanically dislodge stubborn contaminants from submerged surfaces. This sump overflows into the boiling sump completing the loop for the TCA.

CAUTION: The degreaser fume hood is throtled down to a substandard face velocity so as to avoid carrying excessive quantities of the solvent, trichloroethane, up the exaust. Technically, the degreaser is not required to be in a fume hood at all. Please minimize disturbances of the fumes within the tank by moving slowly(<11ft/min). You should also use a neoprene glove over your PVC glove and hold your breath whenever you load and unload the degreaser. You should take about 10 seconds for your hand to descend into the tank and also 10 seconds to return so the vapors are minimally disturbed. Please hold your face shield with your free hand so it can not fall into the tank.

  1. Turn on the ultra-sonic transducers by pressing the U/S button behind the right hand fume hood door. Be sure to close the door afterward.
  2. Load the wafer carrier or cleaning basket on the glass plate. The plate is cleaned before class by the instructor.
  3. Open the lid of the degreaser and prop it up in the rear of the hood while holding your breath. Your instructor can show you how to do this with minimal disturbance of the vapor blanket.
  4. Slowly (<11ft/min) place the wafer carrier or cleaning basket on the screen above the boiling sump, the sump on the left. (#1 in the diagram)

    5. Usually, you should degrease both the wafer and the tweezers together. There is no point in having one clean and the other not since they would contaminate each other.

  5. Wait for the dripping to stop. About 20 seconds.
  6. If there is aluminium on the wafer or parts, then skip to step B.1.12. Aluminium reacts with TCA causing the TCA to become acidic.
  7. Slowly (<11ft/min) place the wafer carrier or cleaning basket on the screen in the ultrasonic sump, the center sump. (#2 in the diagram)
  8. Wait 1 minute in the ultrasonic sump
  9. Slowly (<11ft/min) place the wafer carrier or cleaning basket on the screen above the water separation sump. (#3 in the diagram)
  10. Wait for the dripping to stop and the parts to cool. About one minute. If nobody is going to use the degreaser soon, turn of the ultrasonic transducers by pressing the OFF button below the U/S button. Be sure to close the door.
  11. Slowly (<11ft/min) move the carrier or basket back down into the fumes over the boiling sump. This condenses fresh, pure TCA on the parts to wash away any residues left by the ultrasonic sump. (#4 in the diagram)
  12. Slowly (<11ft/min) place the wafer carrier or cleaning basket back to the screen above the water separation sump to cool. (#5 in the diagram)
  13. Slowly (<11ft/min) remove the carrier or basket after about 20 seconds and unload on the glass plate. (#6 in the diagram)
  14. Replace the lid of the degreaser slowly. Hold your breath.
  15. Wafers should always be squirted with acetone, then IPA, sprayed with D.I., squirted again with IPA, and dried with nitrogen to remove any remaining residues not soluble in TCA.

    16. Acetone and IPA should be used over the "waste acetone and IPA" container.

    Always put the lid back on the waste container when finished.

    Note 1: Acetone dissolves TCA residue. IPA dissolves acetone residue. Water dissolves IPA residue. The final IPA rinse is only for making it easier to dry the wafer.

    Note 2: A special degreaser is dedicated to PR removal. Please use it for the first degreasing after the majority of the PR has been removed using the PR-acetone rinse. 


    *The vapor cleaning procedure just presented is based on 1,1-trichloroethane (TCA) which is no longer available (Clean Air Act of 1995).  There are alternatives to TCA as a degreasing agent (TCE, methylene chloride), although they are potentially carcinogenic.  An alternative degreasing method will be explained in the laboratory

    II. B.2 THE RCA CLEAN

    This cleaning method is the industry standard way to clean wafers. Although every company has its own way of implementing the RCA clean and may have introduced their own proprietary improvements, they have all been significantly influenced by the work of Kern, a chemist at RCA. One of his articles is included at the end of this appendix. Please read it. Below is the recipe for our particular implementation. We substitute sulfuric acid for hydrochloric acid as described in the other article at the end of this appendix. Sulfuric acid has a few safety advantages because its a solid in its pure form where as hydrochloric acid is a gas and the chlorine in hydrochloric acid may be harmful to the ozone layer. Ideally, if we had time, we would use this procedure before EVERY diffusion furnace operation.

    Preliminary Clean

    1. Transfer the wafers to a teflon wafer carrier specifically reserved for the RCA clean.
    2. Securely mount the teflon wafer carrier handle, also reserved for the RCA clean.
    3. The wafers should be degreased before continuing. The SC-1 solution can be prepared during the degreasing procedure.
    4. Etch the wafers for 30 seconds in the 50:1 DI:HF etch.

    Remove residual organics and certain meatals using the RCA standard clean solution 1 (SC-1)

    1. Rinse the quartz tub, temperature sensor and thermometer under the SC-1 acid hood.
    2. Place the tub on the hotplate with the temperature sensor and thermometer inside.
    3. Add 900 ml of deionized water.
    4. Turn on the temperature controller. Verify that its set for 50°C (actual temperature will be 75°C).

      5. NOTE: Whenever handling strong chemicals it is a very good idea to have DI slowly flowing from a faucet first. Not only will it help dilute accidental spills, but it allows you to rinse your gloves without getting the faucet valve contaminated! Always use neoprene gloves over the PVC gloves when handling strong chemicals and rinse them afterward!

    5. Slowly add 180 ml of hydrogen peroxide(30%).
    6. Slowly add 90 ml of ammonium hydroxide (58%). Be sure to rinse the neoprene gloves, the graduated cylinder, and the outside of the chemical containers with DI when finished.
    7. Stir the solution with a thermometer. (Don't break it!)
    8. Slowly place the wafer carrier into the solution.
    9. Occaisionally stir the solution until it has been over 75°C for 10 minutes.

      10. NOTE: While waiting for the temperature to rise it is possible to begin preparation of the SC-2 solution. Do not forget to occaisionally stir and check the temperature of the SC-1 solution!

    10. Turn off the temperature controller after the SC-1 has been at 75-80°C for ten minutes.
    11. Carefully move the wafer carrier to SC-1 DI rinse tank for 20 seconds.
    12. Spray rinse the wafer carrier and as much of the handle as you can without getting your glove wet. At this point a drop of water from a relatively dirty glove could compromise the whole cleaning process.
    13. Move the wafer carrier to the cascade rinse tank for at least 2 minutes. Leave it there until the SC-2 solution is ready.

    Prepare SC-2 solution

    1. Rinse the quartz tub, temperature sensor and thermometer under the SC-2 acid hood.
    2. Place the tub on the hotplate with the temperature sensor and thermometer inside.
    3. Add 960 ml of deionized water.
    4. Turn on the temperature controller. Verify that its set for 80°C.
    5. Slowly add 160 ml of hydrogen peroxide(30%).
    6. Slowly add 55 ml of sulfuric acid.
    7. Occaisionally stir the solution with the thermometer.
    8. When the solution has reached 75°C, continue.

    Strip hydrous oxide

    1. Move the wafer carrier to the 50:1 HF:DI tank for 15 seconds.
    2. Agitate the carrier in the DI rinse tank for 20 to 30 seconds.

    Desorb remaining contaminants

    1. Place the wafer carrier in the hot SC-2 solution for 10 minuites.
    2. Turn off the temperature controller.
    3. Carefully move the wafer carrier to SC-2 DI rinse tank for 20 seconds.
    4. Spray rinse the wafer carrier and as much of the handle as you can without getting your glove wet. Remember, at this point a drop of water from a relatively dirty glove could compromise the whole cleaning process.
    5. Move the wafer carrier to the cascade rinse tank for 5 minutes. Kern recommended 20 minutes.

    Dry the wafers

    1. Verify that the rinser-dryer is set for:
      1. Rinse time = 120 seconds
      2. Dry time = 120 seconds
      3. Resistivity set point = 14 Megaohm-cm
    2. Rinse the special handle for lifting the wafer carrier from one end and switch it with the other handle.
    3. Load the wafer carrier into the rinser dryer and press start.
    4. Wait for the rinser dryer to stop by itself. Use the time wisely (e.g. prepare for the subsequent furnace operation).
    5. Return the wafer carrier to the rinser-dryer after removing the wafers.

    Clean and reorganize the area. If the next days' instructor discovers the tell tale crystals of dried acid, your whole group will loose performance points.

    B.6 B.5  

    ECE 344 home page.

    Written by Dane Sievers - U of Illinois ECE Dept. - dsievers@eceuil.ece.uiuc.edu with inspiration from Mike Fitsimmons.
    E-mail comments and suggestions to ece344@uiuc.edu or use the FEEDBACK FORM.

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